An optical fire detector, including a radiation transmitter and a radiation receiver, which manages without an optical labyrinth and may thus be installed flush in a ceiling, is described in German Patent Application No. DE 199 12 911 C2. Furthermore, the fire detector includes a system, using which soiling of the transparent cover plate of the fire detector may be recognized and, in addition, it may be monitored whether the radiation transmitter and radiation receiver of the fire detector provided for recognizing smoke still operate correctly. The conventional fire detector has the disadvantage that in addition to the radiation transmitter and radiation receiver provided for recognizing smoke, further radiation transmitters and radiation receivers are necessary for recognizing soiling and for function checking. Overall, at least three radiation transmitters and three radiation receivers are thus necessary.
A fire detector having a system, using which it is possible to differentiate between smoke and other foreign bodies in the scattering volume, is described in German Patent Application No. DE 100 46 992 C1. A significant complexity is also necessary in this known fire detector for differentiating between smoke and other foreign bodies, which makes manufacturing of a fire detector of this type more expensive.